The present invention relates generally to infrared thermography and more particularly to an infrared thermography method for determining film cooled heat transfer coefficients and film effectiveness from a single test.
Film cooling is used extensively in modern gas turbines to cool hot gas path components. Film cooling is effected by injecting a cooler fluid onto the hotter surface through holes or slots provided within the surface of the turbine blades or vanes. As high performance turbine engine technologies advance, turbine inlet temperatures are raised to higher and higher values to achieve higher thermal efficiency. These higher temperatures necessitate effective film cooling to protect the turbine components. A competing consideration for the turbine designer is the fact that diversion of pressurized air from the compressor section for cooling purposes decreases the overall efficiency of the engine. Thus, the designer carefully considers the effectiveness of film cooling to keep turbine engine efficiencies as high as possible. For example, the geometry of the cooling holes, incident angles, size, geometry and shape of the turbine blades, etc. are all considerations for the turbine engine designer to take into account in order to maximize cooling efficiency. Towards that end, knowledge of both the heat transfer coefficient and adiabatic wall temperature is necessary for the designer to predict and ascertain the benefits and the level of film cooling induced heat flux reduction, requiring that experimentation must be performed for each geometry under consideration in order to prove the efficacy of each. Such experimentation is costly, both in terms of expenditure of time and money and obviously, any reduction in these costs by improving testing techniques would be desirable.
In the past, the techniques for obtaining film cooled heat transfer coefficients and film effectiveness values traditionally required two different, related experiments wherein the values are obtained separately. Some of the techniques used low resolution methods such as thermocouples and heater foils, mass transfer analogy methods, etc. In the past decade, the most popular technique for obtaining high resolution measurements has been the liquid crystal technique. In this technique, the test article is painted with a coating of thermochromic liquid crystals prior to test. During the test, the crystals display different colors or hues in response to the temperatures encountered. The color change over the duration of the test is then utilized to indicate the temperature ranges encountered by the regions of interest on the test article and these temperature values are utilized to compute the film cooled heat transfer coefficients and film effectiveness values.
One such liquid crystal technique is described in “A method for the simultaneous determination of local effectiveness and heat transfer distributions in three-temperature convection situations” by Vedula and Metzger appearing in ASME paper 91-GT-345. Vedula and Metzger suggested a simultaneous measurement of film cooled heat transfer coefficients and film effectiveness values from a single test using the above described liquid crystal coating approach. This approach proved problematic, however, due to difficulties associated with obtaining both measurements from a single test in light of the relatively slow color change characteristics of the liquid crystals themselves.
In order to solve this problem, resort was made to two, related yet separate, tests to resolve the film cooled heat transfer coefficients and film effectiveness values at every point on the test surface. This two test method has enjoyed some measure of success but, the desirability of determining both values from a single test remains. Since then, several investigators have attempted various techniques to obtain both film cooled heat transfer coefficients and film effectiveness values but have been largely unsuccessful.
A need exists therefore for an improved method for determining film cooled heat transfer coefficients and film effectiveness values. Such a method would desirably enable the simultaneous determination of both values from a single test while avoiding the use of thermochromic liquid crystals for temperature determination and the attendant problems associated therewith.